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Revision: 1.1
Committed: Wed Oct 15 01:02:27 2003 UTC (20 years, 8 months ago) by pcg
Branch: MAIN
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# Content
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131 .IX Title "VPE.PROTOCOL 7"
132 .TH VPE.PROTOCOL 7 "2003-04-15" "0.9" "Virtual Private Ethernet"
133 .SH "The VPE Protocol"
134 .IX Header "The VPE Protocol"
135 .Sh "Anatomy of a \s-1VPN\s0 packet"
136 .IX Subsection "Anatomy of a VPN packet"
137 The exact layout and field lengths of a \s-1VPN\s0 packet is determined at
138 compiletime and doesn't change. The same structure is used for all
139 protocols, be it rawip or tcp.
140 .PP
141 .Vb 3
142 \& +------+------+--------+------+
143 \& | HMAC | TYPE | SRCDST | DATA |
144 \& +------+------+--------+------+
145 .Ve
146 .PP
147 The \s-1HMAC\s0 field is present in all packets, even if not used (e.g. in
148 authentification packets), in which case it is set to all zeroes. The
149 checksum itself is over the \s-1TYPE\s0, \s-1SRCDST\s0 and \s-1DATA\s0 fields in all cases.
150 .PP
151 The \s-1TYPE\s0 field is a single byte and determines the purpose of the packet
152 (e.g. \s-1RESET\s0, \s-1COMPRESSED/UNCOMPRESSED\s0 \s-1DATA\s0, \s-1PING\s0, \s-1AUTH\s0 \s-1REQUEST/RESPONSE\s0,
153 \&\s-1CONNECT\s0 \s-1REQUEST/INFO\s0 etc.).
154 .PP
155 \&\s-1SRCDST\s0 is a three byte field which contains the source and destination
156 node ids (12 bits each). The protocol does not yet scale well beyond 30+
157 hosts, since all hosts connect to each other on startup. But if restarts
158 are rare or tolerable and most connections are on demand, larger networks
159 are possible.
160 .PP
161 The \s-1DATA\s0 portion differs between each packet type, naturally, and is the
162 only part that can be encrypted encrypted. Data packets contain more
163 fields, as shown:
164 .PP
165 .Vb 3
166 \& +------+------+--------+------+-------+------+
167 \& | HMAC | TYPE | SRCDST | RAND | SEQNO | DATA |
168 \& +------+------+--------+------+-------+------+
169 .Ve
170 .PP
171 \&\s-1RAND\s0 is a sequence of fully random bytes, used to increase the entropy of the data
172 for encryption purposes.
173 .PP
174 \&\s-1SEQNO\s0 is a 32\-bit sequence number. It is negotiated at every connection
175 initialization and starts at some random value.
176 .Sh "The authentification protocol"
177 .IX Subsection "The authentification protocol"
178 Before hosts can exchange packets, they need to establish authenticity of
179 the other side and a key. Every host has a private \s-1RSA\s0 key and the public
180 \&\s-1RSA\s0 keys of all other hosts.
181 .PP
182 A host establishes a simplex connection by sending the other host a \s-1RSA\s0
183 challenge containing the random digest and encryption keys (different)
184 to use when sending packets, plus more randomness plus some \s-1PKCS1_OAEP\s0
185 padding plus a random 16 byte id. The destination host will respond by
186 replying with an (unencrypted) \s-1RIPEMD160\s0 hash of the decrypted data, which
187 will authentify that host. The destination host will also set the outgoing
188 encryption parameters as given in the packet.
189 .PP
190 When the source host receives a correct auth reply (by verifying the
191 hash and the id, which will expire after 20 seconds). it will start to
192 accept data packets from the destination host. The protocol is completely
193 symmetric, so to be able to send packets the destination host must send a
194 challenge in the exact same way as already described.
195 .Sh "Retrying"
196 .IX Subsection "Retrying"
197 When there is no response to an auth request, the host will send auth
198 requests in bursts with an exponential backoff. After some time it will
199 resort to \s-1PING\s0 packets, which are very small (8 byte) and lightweight (no
200 \&\s-1RSA\s0 operations). A host that receives ping requests from an unconnected
201 peer will respond by trying to create a connection.
202 .PP
203 In addition to the exponential backoff, there is a global rate-limit on
204 a per-ip base. It allows long bursts but will limit total packet rate to
205 something like one control packet every ten seconds, to avoid accidental
206 floods due to protocol problems (like a rsa key file mismatch between two
207 hosts).
208 .Sh "Routing and Protocol translation"
209 .IX Subsection "Routing and Protocol translation"
210 The vpe routing algorithm is easy: there isn't any routing. Vped always
211 tries to establish direct connections, if the protocol abilities of the
212 two hosts allow it.
213 .PP
214 If the two hosts should be able to reach each other (common protocol, ip
215 and port all known), but cannot (network down), then there will be no
216 connection, point.
217 .PP
218 A host can usually declare itself unreachable directly by setting it's
219 port number(s) to zero. It can declare other hosts as unreachable by using
220 a config-file that disables all protocols for these other hosts.
221 .PP
222 If two hosts cannot connect to each other because their \s-1IP\s0 address(es)
223 are not known (such as dialup hosts), one side will send a connection
224 request to a router (routers must be configured to act as routers!), which
225 will send both the originating and the destination host a connection info
226 request with protocol information and \s-1IP\s0 address of the other host (if
227 known). Both hosts will then try to establish a connection to the other
228 peer, which is usually possible even when both hosts are behind a \s-1NAT\s0
229 gateway.
230 .PP
231 If the hosts cannot reach each other because they have no common protocol,
232 the originator instead use the router with highest priority and matching
233 protocol as peer. Since the \s-1SRCDST\s0 field is not encrypted, the router host
234 can just forward the packet to the destination host. Since each host uses
235 it's own private key, the router will not be able to decrypt or encrypt
236 packets, it will just act as a simple router and protocol translator.
237 .PP
238 When no router is connected, the host will aggressively try to connect to
239 all routers, and if a router is asked for an unconnected host it will try
240 to ask another router to establish the connection.
241 .PP
242 \&... more not yet written about the details of the routing, please bug me
243 \&...